In recent years, for example, in a semiconductor device manufacturing process, it has been reported that a native oxide film on a semiconductor surface results in a variety of defects and therefore it has also been discussed how to realize desirable semiconductor surface by removing the native oxide film.
Moreover, also in the semiconductor device manufacturing process, particular attention has been paid in these several years to the processing for a semiconductor surface utilizing hydrogen atoms, because only hydrogen is assumed as a substantial reducing gas which may be used for the semiconductor device manufacturing process. Especially high efficiency of hydrogen plasma has been confirmed for stripping of a resist used as a mask during ion implantation (S. FUJIMURA, J. KONNO, K. HIKAZUTANI AND H. YANO, JPN. J. APPL. PHYS., 28,2130, 1989) and for elimination of a native oxide film on the surface of a semiconductor device (A. KISHIMOTO, I. SUEMUNE, K. HAMAOKA, T. KOUJI, Y. HONDA AND M. YAMANISHI, JPN, J. APPL. PHYS., 29, 2273, 1990). In this manner, the contribution of hydrogen atoms, for example, is considered essentially in the process for removing a native oxide film on the surface of a semiconductor device. However, such elimination process is carried out mostly in the hydrogen plasma, providing a fear of generating damages.
It is considered to be a result that hydrogen atoms are easily recombinated and immediately return to hydrogen molecules. Therefore, it has long been desired to develop the technique for leading a large amount of hydrogen atoms to the area wherein influence of high energy particles of plasma can be neglected.
As a method of removing a native oxide film, some methods, such as realization of etching of a native oxide film of silicon by a reduced fluoric acid and termination of hydrogen (for example, G. S. HIGASHI ET AL., JPN. J. APPL. PHYS, LETT., 56, 656, 1990) and removal of a native oxide film of silicon or gallium-arsenic by hydrogen plasma (for example, A. KISHIMOTO ET AL., J. APPL. PHYS., 29, 2273, 1990) are well known. Moreover, it has also been reported that a native oxide film on the surface of a semiconductor device can be removed by hydrogen atoms (hydrogen radical) (T. TAKAHAGI ET AL., J. APPL. PHYS., 68, 2187, 1990), (B. Anthony ET AL., J. Vac. Sci. Technol. B7(4), July/August. 1989), (J. Cho ET AL., Appl. Phys. Lett. 59(16), Oct. 14, 1991). In addition, as a typical method which has long been used, hydrogen annealing under the temperature as high as approximately 100.degree. C. has been known.
The existing high temperature hydrogen annealing described above is the most typical method of the prior art, but this method gives rise to difficulties in microminiaturization because it is impossible, for example, to ignore changes of profile of a diffused layer during the annealing. It becomes more distinctive as the microminiaturization progresses.
Moreover, in the existing processing by reduced fluoric acid described above, a stable surface can be obtained, for example, for the plane (111) of silicon, but the stable surface cannot be ensured for the plane (100) and the wet processing makes it difficult to make the connection with processing apparatuses in the successive stages (such as CVD, epitaxial and sputter or the like). Moreover, there arises a problem that fluorine is left at the surface even after the completion of the process.
Next, the hydrogen plasma processing of the prior art explained previously can advantageously overcome the problems described above but still has a problem that high energy particles represented by ions or electrons collide with the surface of semiconductor device resulting in damage. If power is lowered in order to eliminate damage, the processing rate is extremely lowered. This method is not practical because the processing takes too much time in the references listed above.
To prevent the processing rate from lowering, a method in which a small amount of water vapor is added to hydrogen plasma has been proposed. The method is useful for increasing hydrogen atom concentration in the plasma and controlling recombination of hydrogen atoms in the down-flow (Kikuchi, Fujimura, Suzuki, Yano: Fall Meeting of 39th Japanese Applied Physics Association; 29A-ZS-7). FIG. 12 illustrates an example of a conventional down-flow processing apparatus for which the water vapor is added to hydrogen. In FIG. 12, the numerals designate as follows: a quartz (quartz glass is more accurate but it is called only quartz for simplicity) tube 41, a microwave cavity 43 to which microwaves are applied from a microwave power supply 42, and an ESR cavity 44 coupled with an ESR apparatus 45.
However, in the conventional method where water vapor is added to hydrogen plasma, not only hydrogen atoms but also oxidation species such as OH radicals or oxygen atoms are generated though they are a small amount. Particularly, the OH radicals among these elements have an extensive oxidation power and is therefore considered as a factor which impedes the reducing effect of hydrogen atoms.
Moreover, as a method of the prior art for solving the problem that a rate of processing utilizing hydrogen plasma is low, a method in which water vapor and moreover oxygen are added to hydrogen gas, and it is also reported that this method realizes high concentration of hydrogen atoms within the down-flow owing to an increase of the dissociation coefficient of hydrogen molecules in the plasma and the control of recombination of hydrogen atoms in the down-flow (for example, Kikuchi, Fujimura, Suzuki, Yano: Fall Meeting of 39th Japanese Applied Physics Association; 29A-ZS-7).
However, since water vapor and oxygen gas were added to the hydrogen plasma under the condition that the flow rate thereof was fixed in this method, oxygen gas and OH radical were also generated simultaneously in addition to hydrogen atoms. In this case, the rate of adding both water vapor and oxygen gas to the hydrogen gas, and the concentration of oxygen gas and OH radicals are determined uniquely for the maximum concentration of the hydrogen atoms, which restricts freedom of process design. Moreover, the papers by Anthony et al. and Cho et al. mentioned above describe the fact that damages on the Si surface could be eased by removing a native oxide film by hydrogen atoms in the down-flow, but also suggested it would be better to make the residual moisture content as low as possible and also the concentration of hydrogen atoms at the Si surface lower. Therefore, it is difficult to expect remarkable improvement in an efficiency of removing the native oxide film by hydrogen atoms.